Geosynchronous communications satellites and spacecraft have come into widespread use for receiving uplink signals transmitted from ground stations, and for retransmitting downlink signals back to other ground stations. Because of the limited number of geosynchronous "slots" or locations, and because of the very high capital cost of such spacecraft, both in construction and launch, each spacecraft must handle as many "channels" or independent signals as possible.
In order to simultaneously process several independent signals without crosstalk, some kind of multiplexing scheme must be adopted. One of the more popular multiplexing schemes is frequency division multiplexing, in which each independent information signal is transmitted on an RF carrier signal having a frequency different from the frequency of other RF carriers which carry other independent information signals, An advantage of such an arrangement is that, if desired, independent information signals, encoded onto appropriate RF carriers, may be transmitted to the spacecraft from a plurality of different locations on the Earth's surface, whereas some other multiplexing schemes, such as polarization or phase multiplexing, cannot readily be achieved from disparate locations. The communications spacecraft receives the RF signals, preamplifies and filters the received signals as necessary for noise control, and then demultiplexes the signals to separate the RF signals, so that each independent RF signal flows through a different channel or path. Within its path, each RF signal can be further processed, as by amplification. Ultimately, the processed, demultiplexed RF signals are recombined or multiplexed, often in a frequency-translated form, for retransmission back toward Earth.
As a result of the aforementioned high cost of each spacecraft, it is very desirable to continue use of an operating spacecraft for as long a time as possible. While equipment degradation or failure may result in removal of a communication spacecraft from service, the maximum lifetime is determined, in the absence of equipment failures, by the time required to consume the propellant which is used for attitude control and stationkeeping. Consequently, there is an economic incentive to reduce the total weight of the spacecraft, so as to be able to maximize the amount of propellant which can be launched into orbit.
The conventional demultiplexer in a communications spacecraft consists of an array of waveguide filters or resonators, each tuned to a different one of the various signal or RF carrier frequencies, all of which resonators are coupled in parallel to receive the multiplexed signal, and each of which has an output port to which one of the demultiplexed carriers is coupled. In order to avoid excessive signal losses, the waveguide filters must be relatively large, with physical dimensions of at least one quarter wavelength (.lambda./4), but often more. At the frequencies commonly in use for such communications, which range from the radar L-band to X-band, or about 0.5 GHz to 10 GHz, the RF carrier free-space wavelengths range from about twenty-four inches to one inch, respectively. Demultiplexers of this sort tend to be large and heavy. Improved demultiplexers are desired.